A nuclear reactor such as a boiling water reactor (BWR) includes a pressure vessel containing a nuclear reactor core submerged in water which is heated thereby for generating steam. The steam is channeled through a main steamline to a steam turbine which powers a generator for producing electrical power for an electrical utility grid. The output power of the reactor is primarily controlled by a recirculation flow control system (RFCS) which typically includes jet pumps inside the pressure vessel which are powered by external recirculation pumps which selectively control recirculation of the water within the pressure vessel and through the core. Increased recirculation increases the power output of the reactor, and thereby increases the steamflow through the main steamline to the turbine for increasing electrical power from the generator.
Disposed in the main steamline are a plurality of conventional control valves which are selectively openable and closeable for regulating the amount of steamflow to the turbine. Disposed upstream from the control valves is one or more bypass valves which are selectively opened for bypassing a portion of the main steamflow around the control valves and turbine directly to the main condenser of the turbine as required to reduce the rate of reactor pressure rise as the control valves are closed during operation, for example. A pressure control system includes a pressure regulator operatively joined to the control valves and bypass valve for maintaining a predetermined pressure within the reactor in response to the steamflow, or flowrate. A turbine control system includes a turbine controller also operatively joined to the control valves and the bypass valve for controlling flow of the main steam to the turbine in conjunction with the pressure regulator. A low value gate is provided between the pressure regulator and the turbine controller for selecting the lesser of the two demands therefrom to provide a control valve flow demand to the control valves for controlling the operation thereof. Where the control valve flow demand is predeterminedly less than the demand from the pressure regulator, the bypass valve is automatically opened to dump a portion of the main steam into the condenser to prevent undesirable reactor pressure rise, for example.
Associated with each of the pressure regulator and the turbine controller are conventional demand limiters. The limiter for the pressure regulator limits the total steamflow demand collectively through the control valves and the bypass valves to prevent a failure in the control system from causing excessive steamflow from the reactor, also known as blowdown. Blowdown is a conventionally known occurrence which reduces the water level in the reactor and the pressure therein which can lead to conventionally known transition boiling in the fuel bundles therein, which can damage the fuel by overheating. The limiter associated with the turbine controller limits the steamflow through the turbine to prevent damage thereto due to excessive flow.
Faults which cause one or more of the control valves to fall in a closed position reducing or preventing flow therethrough relative to the normally demanded flowrate therefor will necessarily cause the reactor pressure to increase. The pressure regulator will then increase the flow demand to the control valves for reducing the increasing reactor pressure. However, the pressure regulator limiter will prevent the control valves and bypass valve from fully opening since the limit is predetermined for preventing the undesirable blowdown occurrence. If the reactor is operating at a sufficiently high power level, the steamflow through the operable, non-failed control valves and bypass valve will be insufficient to avoid reactor pressure increase, which will then cause the reactor protection system to effect a reactor scram to shut down the reactor. The increased reactor pressure will reduce transition boiling margin which may lead to fuel damage by overheating thereof.
However, the capacity of the operable, non-failed control valves and bypass valve in most boiling water reactors is typically sufficient to accommodate the transient caused by failure of a single control valve without requiring a reactor shutdown. However, this additional capacity is unavailable for use in conventional reactors since the limiters are required to ensure safe operation of the reactor under other modes of operation. The pressure regulator limiter limits the opening magnitude of both the control valves and the bypass valve to prevent undesirable blowdown, for example. And, the turbine controller limiter limits the magnitude of opening of the control valves to prevent excessive flow to the turbine.